1. Field of the Invention
This invention relates generally to communication networks. More specifically, this invention relates to performance monitoring in high-speed packet networks.
2. Description of the Related Art
A number of acronyms well known in the art are used herein. For convenience, they are summarized in Table 1.
TABLE 1ACRONYMMeaningAISalarm indication signalAPSautomatic protection switchingBERbit error rateBIPbit interleaved parityCVcode violationDS-Ndigital signal at level NESerrored secondsES-LFEfar end line errored secondES-SSection Errored SecondLOFloss of framingLOSloss of signalOC-Noptical carrier level at level NOSoperations systemPDHplesiochronous digital hierarchyPOHpayload overheadRDIremote defect indicationREIremote error indicationSDHsynchronous digital hierarchySEFseverely eroded framingSEFseverely errored framingSESseverely errored secondSONETsynchronous optical networkSPEsynchronous payload envelopeSTMsignaling traffic managementSTS-Nsynchronous transport signalsat level NTDMtime division multiplexedTOHtransport overheadUASunavailable seconds. A count of theseconds during which a layer wasconsidered to be unavailable.VTvirtual tributaryVTGvirtual tributary group
High-speed communications networks continue to increase in importance in modern telecommunications. As an example, the Synchronous Optical Network (SONET) is a set of standards that define a hierarchical set of transmission rates and transmission formats for carrying high-speed, time-domain-multiplexed (TDM) digital signals. SONET lines commonly serve as trunks for carrying traffic between circuits of the plesiochronous digital hierarchy (PDH) used in circuit-switched communication networks. SONET standards of relevance to the present patent application are described, for example, in the document Synchronous Optical Network (SONET) Transport Systems: Common Generic Criteria (Telcordia Technologies, Piscataway, N.J., publication GR-253-CORE, September, 2000). While the SONET standards have been adopted in North America, a parallel set of standards, known as Synchronous Digital Hierarchy (SDH), has been promulgated by the International Telecommunications Union (ITU), and is widely used in Europe. From the point of view of the present invention, these alternative standards are functionally interchangeable.
There are four optical interface layers in SONET: path layer, line layer, section layer and photonic layer. These optical interface layers have a hierarchical relationship, with each layer building on the services provided by the lower layers. Each layer communicates with peer equipment in the same layer and processes information and passes it up and down to the next layer by mapping the information into a differently organized format and by adding overhead. In a simplified example, network nodes exchange information as digital signals (DS1 signals) having a relatively small payload. At a source node of the path layer several DS1 signals are packaged to form a synchronous payload envelope (SPE) composed of synchronous transport signals (STS) at level 1 (STS-1), along with added path overhead. The SPE is handed over to the line layer. The line layer concatenates multiple SPEs, and adds line overhead. This combination is then passed to the section layer. The section layer performs framing, scrambling, and addition of section overhead to form STS-Nc modules. Finally the photonic layer converts the electrical STS-Nc modules to optical signal and transmits them to a distant peer node as optical carriers (OC-N signals).
At the distant peer node, the process is reversed. First, at the photonic layer the optical signal is converted to an electrical signal, which is progressively handed over to lower levels, stripping off respective overheads, until the path layer is reached. The DS1 signals are unpackaged, and terminate at the destination node.
The lowest-rate link in the SONET hierarchy is the optical carrier level (OC-1) at the path layer, which is capable of carrying 8000 STS-1 frames per second, at a line rate of 51.840 Mbps. An STS-1 frame contains 810 bytes of data, which are conventionally organized as a block of nine rows by 90 columns. The first three columns hold transport overhead (TOH) while the remaining 87 columns carry the information payload, referred to as the synchronous payload envelope (SPE). The SPE contains one column of payload overhead (POH) information, followed by 86 columns of user data. The POH can begin at any byte position within the SPE capacity of the payload portion of the STS-1 frame. As a result, the SPE typically overlaps from one frame to the next. The TOH of each frame contains three pointer bytes (H1, H2, H3), which are used to indicate where in each frame the POH begins and to compensate for timing variations between the user input lines and the SONET line on which the STS-1 frames are transmitted.
STS-1 frames can efficiently transport DS-3 level signals, operating at 44.736 Mbps. The STS-1 frames themselves are not too much larger than DS-3 frames. When signals at rates below DS-3 are to be carried over SONET, the SPE of the STS-1 frame is divided into sections, known as virtual tributaries (VTs), each carrying its own sub-rate payload. The component low-rate signals are mapped to respective VTs, so that each STS-1 frame can aggregate sub-rate payloads from multiple low-rate links. Multiple STS-1 frames can be multiplexed (together with STS-Nc frames) into STS-N frames, for transmission on OC-N links at rates that are multiples of the basic 51.840 Mbps STS-1 rate.
For the purpose of VT mapping, each STS-1 frame is divided into seven virtual tributary groups (VTGs), each occupying 12 columns of the SPE. Within each VTG, four VT sizes are possible:
The size VT1.5 occupies three columns, each with sufficient bandwidth to transport a DS-1 signal at 1.544 Mbps (i.e., the signal carried on a T-1 line). One VTG can contain four VT1.5 sections.
The size VT2 occupies four columns, providing bandwidth sufficient for an E-1 line.
The size VT3 occupies six columns, providing bandwidth sufficient for a DS-1C signal.
The size VT6 occupies twelve columns, providing bandwidth sufficient for a DS-2 signal.
Mapping of the VTs to the columns of the SPE is specified in detail in the above-noted Telcordia publication GR-253-CORE, at section 3.2.4. It is not necessary that all of the VTs in an STS-1 frame be used to carry lower-rate signals. Unequipped VT sections, i.e., sections that have no service to carry in the SPE, are simply filled with default data. These unequipped sections are assigned a special indication in the VT POH byte V5, bits 5 to 7, known as the signal label. In SDH systems, STM-1 frames are similarly divided into sub-rate payload sections of different sizes, referred to as TU-11, TU-12 and TU-2.
Maintenance criteria are extensively specified in the above-noted Telcordia publication GR-253-CORE to enable the maintenance of the integrity of the network and individual network elements. Maintenance includes the general undertakings of (1) defect detection and the declaration of failures, (2) verification of the continued existence of a problem, (3) sectionalization of a verified problem, (4) isolation, and (5) restoration.
Performance monitoring, to which this application particularly relates, is important and sometimes essential to the conduct of the various above-mentioned tasks in network maintenance. Performance monitoring, as used herein, relates to the in-service, non-intrusive monitoring of transmission quality. Network elements are required to support performance monitoring as appropriate to the functions provided at their respective levels in the network. In large part, performance data is accumulated from information carried in overhead bits. The photonic layer is an exception. There, specified physical parameters are monitored. Network elements are also required to perform self inventory, by which a network element reports information to the performance monitor about its own equipment, as well as adjacency information concerning other network elements to which it is physically or logically connected. The above-noted Telcordia publication GR-253-CORE contains generic performance monitor strategies, discusses various types of performance monitor registers (e.g., current period, previous period, recent period and threshold registers), and defines performance monitor parameters for the various signals which are found in SONET communication.
A principal approach taken in SONET performance monitoring is the accumulation by network elements of various performance monitor parameters based on performance “primitives” that it detects in the incoming digital bit stream. Primitives can be either anomalies or defects. An anomaly is defined to be a discrepancy between the actual and desired characteristics of an item. A defect is defined to be a limited interruption in the ability of an item to perform a required function. The persistence of a defect results in a failure, which is defined to be the termination of the ability of an item to perform a required function. A large number of defects and failures are defined in the above-noted Telcordia publication GR-253-CORE.
Functionally, performance monitoring is performed at each layer, independent of the other layers. However, part of the functional model assumes that layers pass maintenance signals to higher layers. For example, a defect, such as Loss of Signal (LOS) that occurring at the section layer causes an alarm indication signal (AIS-L) to be passed to the line layer, which in turn causes an alarm signal (AIS-P) to be transmitted to the STS Path layer. Thus, an AIS defect can be detected at a particular layer either by receiving the appropriate AIS on the incoming signal, or by receiving it from a lower layer. In consequence, performance monitor parameters at a level are influenced by defects and failures occurring at other levels.
Thresholds are defined for most of the performance monitor parameters supported by SONET network elements. These are used by the performance monitor to detect when transmission degradations have reached unacceptable levels. It is common for hysteresis to be employed before a declared defect or failure can be terminated, in order to assure stability of the system.
Accumulation intervals are defined for each performance monitor parameter. Data accumulated in successive accumulation intervals are required to be independently maintained in a memory as a pushdown stack during a current day's operation. Each network element reports its statuses and results periodically to a higher authority or performance monitor management system. It is the responsibility of the performance monitor management system to derive time-based calculations such as the time during which an defect or failure persisted (errored seconds) and other performance monitor related parameters. Each of the parameters that have to be calculated is depend on one or more variables related to SONET defects, SONET counters, and SONET failures.
For example, Severely eroded seconds at the line level are monitored using the performance monitor parameter SES-L. This parameter is advanced if any of the following SONET defects was active during the previous second: severely eroded framing (SEF), loss of signal (LOS), and alarm indication signal (AIS-L).
As a second example, the counter CV-L counts coding violations at the line level. The performance monitor parameter SES-L is advanced if the SONET counter CV-L is above 9834.
Conventional performance monitor management implementations employ a separate state machine for the monitoring of each performance monitor parameter. An advantage of the state machine approach is the possibility of an “undo” operation, or rollback to a previous state. However, due to the large numbers of performance monitor parameters, and their interrelationships, the state machines are complex, and difficult to maintain and debug. In conventional performance monitor management systems rollback has been accomplished in two ways. In a first approach, it is possible to maintain a previous state for a period of time. During this the system can discard new state data and roll back to the previous state. Maintaining large number of states involves complex software, large amounts of memory storage, and implies slow performance.
A second approach to performance monitor management provides inverse operations. For example, for each increment function, a decrement function can be provided. Using inverse operations requires the propagation of data through the network, under various conditions of operational impairment, and possibly to computer systems outside the network. Considerable system coordination among various network elements and external systems again implies a high degree of design complexity and expense in program maintenance and administration.
U.S. Pat. No. 6,097,702 to Miller et al. discloses an implementation of a performance monitor management system using a library of event-responsive code modules which are installed in various telecommunication equipment as components of event-responsive performance monitoring software. While the use of such modules may mitigate the load on designers of a performance monitor management system, there still remains a need for a more efficient, simpler implementation of a performance monitor management system in a SONET network.